Silicone rubber compositions are used in a wide variety of fields by virtue of their advantages including ease of working, ease of molding, and excellent molded properties such as heat resistance, freeze resistance, weather resistance and electrical insulation. In particular, compositions based on fluorosilicone rubber having 3,3,3-trifluoropropyl groups as side chain have solvent resistance as well and find use as diaphragms, O-rings, oil seals and other parts in transportation equipment and petroleum-related equipment.
However, it is known that fluorosilicone rubber substantially swells and loses its physical strength in alcohol-mixed fuels which are increasingly used in modern vehicles or the like. For example, the rubber is found to experience a substantial loss of strength when immersed in FAM B (test fuel oil consisting of toluene, isooctane, diisobutylene, methanol, ethanol and water) employed in Europe. Efforts to develop fluorosilicones having enhanced durability to alcohol fuel were made in the past as described in Patent Documents 1 to 3. These methods, however, fail to improve the tensile strength of fluorosilicone after immersion in alcohol-containing fuel oil.
In Patent Document 4, the applicant proposed an organopolysiloxane as an effective structural material for use at the site where heat resistance and oil resistance are required. This organopolysiloxane alone is still insufficient as a structural material durable to alcohol-containing fuel oil.